Warlock – “Cultist” [Campaign 4, Mission 3]

Yes, there was indeed a 3rd demon. Yesterday, after a long break, I easily completed this mission. Well... I hired heroes with high levels so that they wouldn't fight against me later. I didn't build up the towns much. And, most importantly, I killed the red demons with Shadia. She takes them down like it's nothing. Although I tried to kill them with Raileigh, but for some reason, it didn't work.
Here, tell me, friends. I've been wondering about something for a long time. How can I split, for example, 20 assassins into 10 and 10? Someone already asked this question in this forum, and they answered that you need to press Shift. But I press it, and nothing happens.